The present invention concerns an improved method for the preparation of gliadin- and glutenin-rich fractions out of gluten in an aqueous medium and in the presence of an acid.
In the patent and scientific literature, several methods have already been described for the separation of wheat gluten into gliadin- and glutenin-rich fractions. Also the use of these fractions has already been the subject of a number of patent and scientific publications.
On a laboratory scale, many techniques using different solvents and other conditions have been studied to separate the two fractions. In the book “Protéines végétales” (1985), p. 161-210, Popineau reported a detailed review of these techniques but, in most cases, these techniques can not be extrapolated to an industrial scale because they generally use solvents which are not allowed in food preparation and they include a step such as preparative chromatograph which, although suitable for pharmaceutical usage, is too expensive to be used in the food industry.
Among the solvents allowed for use in the preparation of food components, water-alcohol mixtures and acetic acid solutions were tested at a laboratory scale for their ability to separate the protein fractions.
In EP 685 164, an extraction method is described by which an aqueous solution of ethanol having a concentration of 30 to 70% by volume, an aqueous solution of isopropyl alcohol or n-propanol having a concentration of 10 to 20% by volume, or an aqueous solution of acetone having a concentration of 20 to 50% by volume is used to prepare a fraction having a gliadin concentration of as high as 80% or above. The extraction can be performed by means of an acidic aqueous solution of ethanol having a concentration of 5 to 30% by volume and a ph of 3.5to 5.5 also, to give a gliadin fraction having a concentration of 50% or above. Among the acids which can be used belong acetic, citric, malic, lactic, adipic, fumaric, tartaric, gluconic, phosphoric and phytic acid.
Because the use of flammable solvents requires some additional saftty measures, it is considered as advantageous to have a process where no such solvents are needed. Such a process is described in “Industries Alimentares et Agricoles” (1974) by C. de Meester, by which 0.01-0.1 M acetic acid solutions are used to extract proteins from vital wheat gluten. After an extraction period ranging from 1 to 8 hours, the final yields are 20-55%. The soluble fraction, the gliadin-rich one, was recovered by precipitation at neutral pH. Because the insolubilised gliadins are very sticky, such a technique is not very suitable for industrial application and the protein precipitation stage must be avoided in the recovery process. This method was elaborated on lab scale, but is not workable on an industrial scale.
An other solvent-free process is described in WO 9710260. This application describes a method for fractionating wheat gluten. According to this method, wheat gluten is first dispersed in an aqueous acidic medium at a first acidic pH in the presence of a reducing agent, to reduce disulphide bonds in the gluten protein. Then the pH of the dispersion is raised to a second level above said first pH, causing glutenin to precipitate while leaving gliadin suspended in the dispersion and finally the glutenin and gliadin are separated into the respective fractions.
Also EP 992 193 refers to an aqueous extraction method by which an aqueous solution of pH smaller or equal to 4.5 and comprising 0.1 to 10% weight/volume of at least one organic acid is used. Among the acids that can be selected there is mentioned: citric, lactic, malic, acetic acid, but also phosphoric acid and salts thereof. This application provides no further details on how such an aqueous extraction is performed.
By Bérot et al., in “Intern. Journal of Food Science & Technology” (1994), p. 489-502, a pilot scale process for fractionating gluten, using acid aqueous solutions is described. According to this publication, wheat gluten and a dilute acid solution are mixed in a high shear mixer for two minutes, and the stirred continuously for 30 minutes. The aqueous solvent to gluten ratio varied between 7:1 and 16:1 (v/w). The soluble gliadin-rich fraction was separated by means of a Westfalia horizontal centrifuge decanter and the residue was then again mixed with water or with dilute acid and submitted to a second centrifugation step. This resulted in an insoluble glutenin-rich fraction and the intermediate fraction. The overall characteristics of the intermediate fraction were not far from these of wheat gluten.
This process, although providing a gliadin-rich fraction, is less attractive because the intermediate fraction is obtained as a by-product. This by-product is considered as an important disadvantage with regard to the yield.
Also the processing set-up is quite complicated and deserves to be simplified, preferably without loss of functional properties of the fractionated material.
Apart from the complicated nature of the Bérot-process, it is observed that the intermiediate fraction can not be reprocessed via back-mixing with fresh gluten. Indeed, it was observed that re-circulation of this intermediate product resulted in the reduction in purity and quality of the fractions.